The main objective of the proposed study is to identify the sites and mechanisms of synthesis, assembly, and degradation of plasma membrane components using the avian salt gland as a model system. Thus far it has been established that osmotic stressing induces secretory cells in the salt gland to greatly amplify their plasma membrane to a maximum extent in about 7 days. Accompanying this is an increase in Na, K-ATPase activity and content in the membrane. Membrane synthesis is preceded by DNA, RNA and protein synthesis (beginning 2 hrs. after osmotic stress) and paralleled by greatly increased leucine and fucose incorporation. A single-cell suspension of secretory cells of high quality, biochemically and morphologically, has been prepared which will be used for pulse-chase autoradiography experiments designed to trace the route of membrane synthesis. Ouabain binding, and 3H-ouabain autoradiography have been used to localize and quantitate the Na, K-ATPase (a primary surface membrane marker) during membrane synthesis and turnover. With regard to turnover, ultrastructural and biochemical studies show that the excess membrane is lost when salt-stressed ducks are again given freshwater. Na, K-ATPase content falls off rapidly during "destressing" and ultrastructural studies confirm that the destressing process requires about 7 days. Preliminary studies on rates of DNA synthesis and DNA content, on protein content of the gland, and on distribution of hydrolases during destressing may indicate that while some membrane turnover is due to cell replacement, much of the turnover is due to internalization and digestion within secretory cells. Currently cytochemically useful ouabain derivatives are being synthesized to test the internalization hypothesis.